EP 0 614 649 B1 describes a polyaxial bone screw with a receiver member with a nut to be screwed onto the receiver member for securing a rod.
If the thread of the receiver member and of an inner screw is a metric thread, force components in radial direction of the cylindrical receiver member occur when screwing in the inner screw, which could cause splaying of the legs of the receiver member resulting in a loosening of the inner screw.
Implants are known which employ only an inner screw for fixation of the rod, whereby a specific shape of the thread is provided for reducing the forces acting radially outward when screwing in. For example, U.S. Pat. No. 5,005,562 describes an implant having a receiver member, wherein the shape of the thread of the receiver member and the inner screw is formed as saw tooth thread, whereas WO 00/27297 describes the use of a thread with a negative angle of the load flank.
Such implants that have thin side flanks of the receiver member, however, encounter a problem when using only an inner screw for fixation without an additional securing via a nut to be screwed on or a cap covering the legs of the receiver member at the outside or a ring or the like. The problem is illustrated by means of FIG. 7, which shows schematically a polyaxial bone screw similar to that shown in EP 0 614 649 B1, but without the external cap or nut. As shown in FIG. 7, there is a screw element 101 having a spherical segment-shaped head 102, which is held in a receiver member 103 having a U-shaped recess for insertion of the rod 100. A pressure element 104 acts upon the spherical segment-shaped head 102 and, for fixation of the rod and of the head, an inner screw 105 with a metric thread is provided which can be screwed into the receiver member 103. At the time of final tightening of the inner screw with high torque, the inner screw tilts about the rod support surface or rotates about the rod such that a torsional force acts upon the legs of the receiver member distorting the same against each other. This results in an asymmetric splaying and deformation of the thread receiving parts resulting in the possibility that the inner screw can slide out of the lower left and the upper right turn shown by a circle in FIG. 7, respectively, and the respective turn may be skipped.
To avoid this, the inner screw should be tightened with reduced torque which, however, impairs the retention force. Further, the problem also can be reduced, if a wall of the receiver member is very thickly dimensioned, which is, however, an obstacle to the requirement of a compact implant design.
The problem of tilting of the inner screw at the time of final tightening which has been described is independent from the form of the used thread.
It is desirable to provide an element of the type described which allows a reliable fixation of the rod and at the same time makes a compact design possible.